Position and Orientation Tracking of Transponder

ABSTRACT

One or more implementations are described herein for improved instrument tracking. In a surgical navigation system, one implementation stores a plan for an image guided procedure, before conducting the procedure. This plan includes a path to be traversed by a medical instrument during the procedure. An image of the patient&#39;s anatomy displayed with a superimposed a pictorial representation of the path on the image. A transpoder coupled to the medical instrument and emits a signal while inside the patient&#39;s body. A position and/or orientation of the transponder (and the instrument) is determined based, at least in part, upon the received transponder signal.

BACKGROUND

The present invention generally relates to an electromagnetic tracking system. In particular, the present invention relates to an electromagnetic tracking system using a single-coil wired or wireless transmitter.

Many medical procedures involve a medical instrument, such as a drill, a catheter, scalpel, scope, stent or other tool. In some cases, a medical imaging or video system may be used to provide positioning information for the instrument, as well as visualization of an interior of a patient. Typically, during the course of a procedure, an instrument is guided by continuously obtaining and viewing x-ray images that show the current location of the instrument along with a portion of the patient's anatomy in a region of interest. However, because repeated exposure to x-ray radiation is harmful to medical personnel that perform image guided procedures on a daily basis, many navigation systems have been proposed that attempt to reduce exposure to x-ray radiation during the course of a medical procedure.

For example, electromagnetically tracking the position of medical instruments during a medical procedure is used as a way to decrease exposure to x-ray radiation by decreasing the number of x-ray images acquired during a medical procedure. Typically, an electromagnetic tracking system employs a transmitter coil, a transponder coil, and a receiver coil. The transmitter coil emits a signal at a frequency that is picked up by the transponder coil. The transponder coil emits a signal at the same frequency in response to the transmitter signal. The signal from the transponder is received at the receiver coil and the tracking system calculates position information for the medical instrument with respect to the patient or with respect to a reference coordinate system. During a medical procedure, a medical practitioner may refer to the tracking system to ascertain the position of the medical instrument when the instrument is not within the practitioner's line of sight.

The tracking or navigation system allows the medical practitioner to visualize the patient's anatomy and track the position and orientation of the instrument. The medical practitioner may then use the tracking system to determine when the instrument is positioned in a desired location. Thus, the medical practitioner may locate and operate on a desired or injured area while avoiding other structures with less invasive medical procedures.

Tracking systems are also used outside of the medical field to track the position of items other than medical instruments. For example, tracking technology is used in forensic and security applications. Retail stores use tracking technology to prevent theft of merchandise. In such cases, a passive transponder can be located on the merchandise. A transmitter may be strategically located within the retail facility. The transmitter emits an excitation signal at a frequency that is designed to produce a response from the transponder. When merchandise carrying a transponder is located within the transmission range of the transmitter, the transponder produces a response signal that is detected by a receiver. The receiver then determines the location of the transponder based upon characteristics of the response signal.

Tracking systems are also often used in virtual reality systems or simulators. For example, tracking systems are used to monitor the position of a person in a simulated environment. A transmitter emits an excitation signal and a transponder located on the person produces a response signal. The response signal is detected by a receiver. The signal emitted by the transponder is then used to monitor the position of a person or object in a simulated environment.

Electromagnetic tracking systems such as those presented above may employ coils that act as the transmitters, transponders, and receivers. Typically, an electromagnetic tracking system is configured in an industry-standard coil architecture (ISCA). ISCA uses three colocated orthogonal quasi-dipole transmitter coils and three colocated quasi-dipole receiver coils. Other systems may use three large, non-dipole, non-colocated transmitter coils with three colocated quasi-dipole receiver coils. Another tracking system architecture uses an array of six or more transmitter coils spread out in space and one or more quasi-dipole receiver coils. Alternatively, a single quasi-dipole transmitter coil may be used with an array of six or more receivers spread out in space.

The ISCA tracker architecture uses a three-axis dipole coil transmitter and a three-axis dipole coil receiver. Each three-axis transmitter or receiver is built so that the three coils exhibit the same effective area, are oriented orthogonally to one another, and are centered at the same point. An example of a dipole coil trio with coils in X, Y, and Z directions spaced approximately equally about a center point is shown in FIG. 4. If the coils are small enough compared to a distance between the transmitter and receiver, then the coil may exhibit dipole behavior. Magnetic fields generated by the trio of transmitter coils may be detected by the trio of receiver coils. Using three approximately concentrically positioned transmitter coils and three approximately concentrically positioned receiver coils, for example, nine parameter measurements may be obtained. From the nine parameter measurements and a known position or orientation parameter, a position and orientation calculation may determine position and orientation information for each of the transmitter coils with respect to the receiver coil trio with three degrees of freedom.

As discussed earlier, the response signal emitted by the transponder and the excitation signal emitted by the transmitter are incident upon the receiving coil. Typically, in a tracking system using a passive transponder, the excitation signal is much larger than the response signal when both signals are received at the receiver. Because the response signal is emitted at the same frequency as the excitation signal and the response signal is much smaller than the excitation signal, accurately separating and measuring the response signal is difficult.

While current ISCA architectures track a trio of transmitter coils with a trio of receiver coils, many instruments, such as catheters or flexible ear, nose and throat instruments, require a single small coil to be tracked. There is no known conventional electromagnetic tracking system for tracking an instrument using a single coil.

Additionally, with multiple instruments each containing single coil transmitters emitting signals at the same frequency as the transmitter signal, it becomes difficult to discern one transponder signal from another. Thus, if multiple instruments are used simultaneously during a procedure, it becomes difficult to simultaneously track and identify each instrument. There is no known conventional electromagnetic tracking system for tracking an instrument using a single coil that allows for identification and location of the individual coils.

Additionally, to optimize guidance of medical instruments using transponders and receivers, and reduce trauma to a patient, it may be desirable to predetermine a path to be traversed within a patient's anatomy. If the path and therapeutic materials could be entered and saved in a navigation system, medical personnel could access the information regarding the predetermined path and guide a medical instrument accordingly. There is no known conventional surgical navigation system that allows for a pre-operative plan to be entered and saved before beginning a procedure and can be viewed and followed during the course of a procedure.

Thus, there is a need for an improved electromagnetic tracking system using a single-coil wired or wireless transmitter.

BRIEF SUMMARY

One or more implementations are described herein for improved instrument tracking. In a surgical navigation system, one implementation stores a plan for an image guided procedure, before conducting the procedure. This plan includes a path to be traversed by a medical instrument during the procedure. An image of the patient's anatomy displayed with a superimposed a pictorial representation of the path on the image. A transpoder coupled to the medical instrument and emits a signal while inside the patient's body, A position and/or orientation of the transponder (and the instrument) is determined based, at least in part, upon the received transponder signal.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 illustrates a wireless tracker used in accordance with an embodiment of the present invention.

FIG. 2 shows a printed circuit board used in accordance with an embodiment of the present invention.

FIG. 3 depicts a flow diagram for a method for a position, orientation and gain determination used in accordance with an embodiment of the present invention.

FIG. 4 illustrates a dipole coil trio used in accordance with an embodiment of the present invention.

FIGS. 5 and 6 depict a flow diagram for a method for improved instrument tracking in a surgical navigation system used in accordance with an embodiment of the present invention.

The foregoing summary, as well as the following detailed description of certain embodiments of the present invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, certain embodiments are shown in the drawings. It should be understood, however, that the present invention is not limited to the arrangements and instrumentality shown in the attached drawings.

DETAILED DESCRIPTION

Certain embodiments of the present invention provide a system and method for electromagnetic tracking using a single-coil transponder. The system includes a single coil transponder emitting a signal, a receiver receiving a signal from the single coil transponder, and electronics for processing the signal received by the receiver. The electronics determine a position of the single coil transponder.

The transponder may be a wireless or wired transponder. The single coil of the transponder may be a dipole. The transponder may be battery-powered. Additionally, the transponder may be driven with a continuous wave signal.

The receiver may be a printed circuit board. Additionally, the receiver may be a twelve-receiver array. In an embodiment, the receiver may be a twelve receiver circuit printed circuit board. Four circuits may include single spiral coils. Eight circuits may include pairs of spiral coils.

The electronics may identify the transponder and determine position, orientation, and/or gain of the transponder. The electronics may determine a ratio of mutual inductance between the transponder and the receiver to determine the position of the transponder. The electronics may also determine a ratio of currents and/or magnetic fields produced at the transponder to determine the position of the transponder.

Certain embodiments provide an improved instrument tracking system including a single-coil wireless transponder, a printed circuit board receiver array including a plurality of coils and coil pairs, and tracker electronics for analyzing parameter(s) between the transponder and the coils and coil pairs of the receiver array to determine a position of the transponder in relation to the receiver array. The parameters may include mutual inductances and/or magnetic fields. The tracker electronics may also determine a gain and/or an orientation of the transponder. In an embodiment, reciprocity allows the coils of the receiver array to be treated as transponder coils. The system may also include a calibration coil for calibrating the receiver array.

In an embodiment, the printed circuit board receiver array produces magnetic fields as follows: a mostly uniform field point in an X direction; a field varying mostly with X, pointed in the X direction; a field varying mostly with Y, pointed in the X direction; a field varying mostly with Z, pointed in the X direction; a mostly uniform field pointed in the Y direction; a field varying mostly with X, pointed in the Y direction; a field varying mostly with Y, pointed in the Y direction; a field varying mostly with Z pointed in the Y direction; a mostly uniform field pointed in the Z direction; a field varying mostly with X pointed in the Z direction; a field varying mostly with Y pointed in the Z direction; and a field varying mostly with Z pointed in the Z direction.

Certain embodiments provide a method for improved instrument tracking. The method includes driving a transponder coil at a certain frequency to emit a signal and receiving the signal at an array of receiver coils. The method also includes determining a gain of the transponder coil and measuring a mutual inductance between the transponder coil and an array of receiver coils. An initial estimate of a position of the transponder coil is selected. The initial estimate is adjusted using an error-minimizing routine based on the mutual inductance. The initial estimate may be a previous calculation result.

The method may also include calibrating the array of receiver coils. Additionally, the method may include eliminating a sign ambiguity of the gain of the transponder coil. A transponder current may also be determined from the signal received at the array of receiver coils.

In a certain embodiment, a method for electromagnetic tracking includes driving an array of coils at different frequencies, determining ratios of currents produced by the different frequencies, measuring voltages generated at the different frequencies, and calculating ratios of mutual inductances between the array of coils and a single coil located remotely from said array of coils. The method further includes estimating an initial value for at least one of position, gain, and orientation of the single coil and determining a best fit value for at least one of the position, gain, and orientation of the single coil based on the initial value and the ratios of mutual inductances. The method may also include calibrating the array of coils.

FIG. 1 illustrates a wireless tracker 100 used in accordance with an embodiment of the present invention. The wireless tracker 100 includes a transmitter 110, a wireless transponder 115, a receiver 120, and tracker electronics 130. The transmitter 110 emits a transmitter signal. The wireless transponder 115 receives the transmitter signal and emits a transponder signal. The transponder signal may include data such as identification information that may be used to associate a transponder signal with a particular transponder. The receiver 120 detects the transmitter signal and the transponder signal. The tracker electronics 130 analyzes the signals received by the receiver 120 to identify the transponder 115 and determine a position of the transponder 115.

In an embodiment, the transponder 115 is a single-coil wireless transponder. The wireless transponder 115 may be a battery-powered wireless transponder or a passive transponder. Alternatively, a single-coil wired transponder may be used in place of or in addition to the wireless transponder 115.

During some medical procedures, portions of medical instruments may be obscured or covered by portions of a patient's anatomy. For example, a small incision may be made in a patient's abdomen and a medical instrument such as a needle and trocar inserted in the incision. After the needle and trocar is inserted through the incision, the surgeon can not see the portion of the needle and trocar that is within the patient's abdomen.

In order to guide the tip of the needle to a desired region of interest, a transponder may be placed near the tip of the needle. A transmitter can emit a transmitter signal that propagates through the patient's anatomy. The transmitter signal impinges upon the transponder located on the tip of the needle. In response, the transponder emits a transponder signal. The transponder may include a memory that stores data such as identification information that distinguishes the transponder from other transponders. When the transponder receives the transmitter signal, the transponder emits a transponder signal that may include a portion of the data stored in the memory.

A receiver receives the transponder signal. A tracking system coupled to the receiver processes the transponder signal. If the transponder signal contains identification data, the tracking system can identify from which transponder the transponder signal was emitted. The tracking system can also use the transponder signal to calculate the location of the transponder. Consequently, the transmitter, transponder, receiver, and tracking system can be used to identify and locate portions of medical instruments during a medical procedure and to aid in navigating the medical instruments to regions of interest.

In an embodiment, the transponder may be a 23 mm glass transponder with a read only memory of 64 bits and an operating frequency of 134.2 kHz as manufactured by Texas Instruments. The 64 bit memory can be used to store unique identification data that identifies the medical instrument to which the transponder is attached and to distinguish the transponder and its corresponding medical instrument from other transponders and their corresponding medical instruments.

In an embodiment, the transponder coil is small enough that the coil acts sufficiently like a dipole for tracking purposes. A dipole may be described by position, orientation, and gain (or strength). The position, orientation, and strength of the coil may be determined as described below. Therefore, the position, orientation, and gain of the wireless transponder coil and the tracker electronics 130 may be determined without characterization.

In an embodiment, the receiver 120 is a single 0.48 meter by 0.52 meter printed circuit board (PCB). The PCB may include 20 coils formed by copper tracks in the PCB, for example. The coils may be connected in series pairs and/or used individually, for example. In an embodiment, twelve separate conducting paths may be present on the PCB (called the ANT-009 design). PCB coils may be precisely made at a low cost. The ANT-009 PCB may be used as an array of transmitters or as an array of receivers, for example. FIG. 2 shows an embodiment of the ANT-009 PCB.

A transponder 115 with a driver may be used in place of a transponder 115 and transmitter 110 combination. Rather than the transponder 115 emitting a transponder signal after receiving a transmitter signal from a transmitter 110, the transponder driver may be used to provide a signal to the transponder 115 and cause it to emit a transponder signal.

In an embodiment, receiver coils in the PCB are spread out or distributed on the PCB. The distributed coils are susceptible to electrostatic pickup. A Faraday shield may be used to block electrostatic pickup from the PCB without affecting electromagnetic signals received by the receiver 120.

Mutual inductance may be used in the electromagnetic tracking system to identify the positions of components in the system. Mutual inductance may allow the system to be divided into two parts: coils and electronics 130. Determining mutual inductance involves a physical design of the coils and a geometrical relationship between the coils but not details of the electronics 130 used to measure the mutual inductance. Additionally, mutual inductance does not depend on which coil receives an applied current.

In addition to the electronics 130 used to measure mutual inductance, a system including one transponder coil and one receiver coil forms a four-terminal two-port network. A varying current injected into one coil induces a voltage in the other coil. The induced voltage V is proportional to the rate of change of the applied current I: V=L _(m)(dI/dt)  (2), wherein L_(m) represents mutual inductance. L_(m) is based on the geometry of the coils (closed circuits). L_(m) is a ratio independent of applied current waveform or frequency. Thus, L_(m) is a well-defined property that may be measured with reasonable precision.

The position, orientation, and gain (POG) of the transponder 115 may be calculated with respect to a coordinate system of the receiver 120. POG determinations employ reciprocity to generate magnetic field models that treat PCB receiver coils as transponder coils. Reciprocity indicates that a mutual inductance of a pair of coils is independent of which coil is driven. By using pairs of coils in series on the PCB, magnetic fields in XYZ directions and with XYZ gradients are formed in a “sweet spot” in relation to the PCB. For example, fields are formed 0.1-0.2 meters above the center of the PCB. In an embodiment, the PCB includes 12 distinct single coils and coil pairs. A variety of magnetic fields enhance numerical stability of the POG calculation.

In an embodiment, the gain of the single transponder coil may be determined with 6 or more receiver coils. In an embodiment, a mutual inductance model provides 12 mutual inductances from the transponder coil to each of the receiver coils as a function of POG. First, an initial estimate of POG may be selected. For example, a POG result from a previous measurement and calculation cycle may be used as an initial estimate or seed for a POG calculation. Then, an error-minimizing routine may be used to adjust the POG estimate. The POG estimate is adjusted to minimize a difference between measured and modeled mutual inductances.

If a sine wave is emitted by the transponder 115 and the receiver 120 calculation is phase-locked to the transponder signal, a sign of the transponder coil gain may not be determined. An unknown sign of the transponder gain may create ambiguity in the POG. For example, reversing the transponder coil end-for-end has no effect on the POG. In an embodiment, tracking may start with the transponder coil at an approximately determined POG. The POG may then be tracked from cycle to cycle.

In an alternative embodiment, sign ambiguity of the transponder gain may be eliminated. A phase or sign of the transponder 110 sine wave may be determined directly with no memory (e.g., without previous calculations). The phase may be determined without a phase-locked loop.

A complex transponder current (tx_current) may be expressed as a product of two factors: tx_current=tx_current_magnitude*tx_current_phase  (3), where tx_current_magnitude is a magnitude of the transponder 115 current, and tx_current_phase is a phase of the transponder 115 current. In an embodiment, the magnitude of the transponder 115 current is real, positive, and varies slowly. The magnitude of the transponder current is proportional to the gain of the POG. Thus, transponder current magnitude may be determined by a POG calculation. The transponder current phase is a complex, unity magnitude value. The phase is recalculated from newest receiver 120 signal data for each cycle. Transponder current phase may be different for each cycle's data.

In an embodiment, the largest magnitude received signal in a 12-receiver array is one of receivers 0, 5, and 11 of an array of 0 to 11. The three receiver coil boards 0, 5, and 11 have approximately orthogonal directional responses. That is, if the total signal is a reasonable size, at least one of the receiver boards 0, 5, and 11 receives a signal that is not small. For a receiver signal array, receiver signals 0, 5, and 11 may be tested to determine which receiver signal is largest in magnitude. The signal with the largest magnitude is designated receiver_signal[r].

A denormalized transponder current phase may then be calculated as follows: $\begin{matrix} {{{{tx\_ current}{\_ phase}{\_ denormalized}} = {{sign}\frac{{receiver\_ signal}\lbrack r\rbrack}{i\quad 2\pi}}},} & (4) \end{matrix}$ where the sign is either +1 or −1. Then the current phase may be normalized and the sign corrected: $\begin{matrix} {{{tx\_ current}{\_ phase}} = {\frac{{tx\_ current}{\_ phase}{\_ denormalized}}{{{tx\_ current}{\_ phase}{\_ denormalized}}}.}} & (5) \end{matrix}$ A transponder 115 complex current may then be determined: tx_current=tx_current_mag*tx_current_phase  (6).

Without a second harmonic signal measurement, a sign may be chosen for each cycle to maintain a consistent sign of the receiver_signal[n] elements over time. In an embodiment, tracking of the transponder 115 begins from a selected position, such as a calibration position, to make an initial sign choice (+ or −). A second harmonic current of the transponder coil may be generated with an asymmetrical waveform including even harmonics and a CW fundamental frequency. For example, a transponder coil driver may output an asymmetrical square wave voltage (for example, ⅓, ⅔ duty cycle) to drive the coil in series with a tuning capacitor. Alternatively, a diode (or a series combination of a diode and a resistor, for example) may be connected in parallel with the coil to generate even harmonics.

A harmonic frequency may be used to determine the sign of the fundamental frequency. The harmonic may be amplitude modulated with low-speed analog or digital data without affecting a tracking function. The data may be characterization data, data from a transducer mounted on the transponder 115, or other data, for example.

In an embodiment, a low cost battery-powered transponder driver and coil may be used. Cost may be reduced by not characterizing the single coil of the transponder 115. The low cost driver and single coil may be used in disposable applications, for example.

If a transponder unit 115 is sealed, such as in medical applications, activating or turning a unit “on and off” may present difficulties. In an embodiment, a transponder driver includes a silicon CMOS chip with an on-off flip-flop or latch circuit and a photocell. A brief flash of light sets the flip-flop and activates the driver. Once set, the flip-flop remains set independent of illumination until a specific electromagnetic pulse resets the flip-flop and turns the driver off. After manufacture and testing, the driver-coil assembly may be packaged in a sealed, lightless container, such as a container used for photographic film. The packaged driver is turned off by applying an electromagnetic pulse. When a user opens the package, ambient light turns on the driver. The driver runs until receiving an electromagnetic pulse or until energy in a driver battery is exhausted.

The transponder 115 may be driven by an oscillator powered by direct current, for example. In an embodiment, the wired transponder driver may be powered from a source of 3 volts at a milliampere direct current. For example, photocells powered by ambient light may power the driver. Alternatively, radio frequency energy may be rectified to power the driver.

In one embodiment, a single transponder coil is located at the tip of a catheter. A small silicon photocell is connected across the coil. The photocell is illuminated with amplitude-modulated light. The photocell powers a driver for the transponder coil. Alternatively, two photocells may be connected in antiparallel across the transponder coil. By alternately illuminating each photocell, an alternating current may be generated in the coil.

Alternate illuminations may be achieved using two optical fibers (one to each photocell). Illumination may also be achieved using one fiber to illuminate the photocells through filters of different polarizations or different colors, for example. In another embodiment, two photocells may be integrated on top of each other. Each photocell may be sensitive to different wavelengths of light.

An optically powered coil may have advantages over an electrically powered coil. For example, optical fibers may be smaller than electrical wires. Additionally, a catheter, for example, with an optically powered coil has no electrical energy in most of the length of the catheter. An electrically powered coil may result in some electrical energy in the catheter.

In another embodiment, the receiver 120 may include an array(s) of three-axis dipole wire-wound coil trios. Due to inaccuracies in coil winding, the receiver 120 is characterized before use in tracking. The wire-wound receiver coil arrangement may have a better signal-to-noise ratio than a PCB coil, due to a larger volume of copper in a wound coil of a given volume. Additionally, POG seed algorithms may be used with characterized receiver coils.

In an alternative embodiment, a battery-powered wireless transponder driver receives a clock signal from the tracker electronics 130 via a magnetic, radio frequency, ultrasonic, or other signal generator. A clock signal may eliminate phase-locking and ambiguity in the sign of the transponder gain.

In another embodiment, the wireless transponder 115 may be combined with various wireless radio frequency identification (RFID) schemes. RFID techniques allow for identification and/or data transfer without contact between the transponder 115 and the receiver 120. The wireless transponder 115 may be used with RFID technology to transmit data to the receiver 120 and tracker electronics 130.

As described above, a PCB may be used in an electromagnetic tracking system, such as the wireless tracker 100. The following discussion illustrates an embodiment of the PCB in more detail. The PCB may be configured as a transponder coil array and be used to track a single receiver coil against an array of twelve transponder coils, for example. The PCB may also be configured as a receiver coil array and used to track a single-coil transponder. The PCB may be used as the receiver 120 in the wireless tracker 100 tracking the single-coil transponder 115. Reciprocity allows coils in the receiver coil array to be treated as transponder coils.

In an embodiment, the PCB is precisely manufactured, so a magnetic field model of the PCB may be determined with sufficient accuracy without characterization. A single coil transponder is small enough to be modeled with sufficient accuracy as a dipole with a position, orientation, and gain that are determined through tracking without characterization. In an embodiment, the PCB does not include curved traces. Magnetic fields may be more precisely calculated with straight line segments.

The PCB board, such as the ANT-009 coil board described above and shown in FIG. 2, may facilitate tracking around a small volume “sweet spot” located over the center of the PCB. In an embodiment, the board provides magnetic fields in the sweet spot that are approximately as follows:

1. a mostly uniform field pointed in the X direction;

2. a field varying mostly with X pointed in the X direction;

3. a field varying mostly with Y pointed in the X direction;

4. a field varying mostly with Z pointed in the X direction;

5. a mostly uniform field pointed in the Y direction;

6. a field varying mostly with X pointed in the Y direction;

7. a field varying mostly with Y pointed in the Y direction;

8. a field varying mostly with Z pointed in the Y direction;

9. a mostly uniform field pointed in the Z direction;

10. a field varying mostly with X pointed in the Z direction;

11. a field varying mostly with Y pointed in the Z direction; and

12. a field varying mostly with Z pointed in the Z direction.

The X and Y directions are in the plane of the PCB. The Z direction is perpendicular to the plane of the PCB.

In an embodiment, the ANT-009 coil PCB includes twelve separate electrical circuits. Four of the circuits include single spiral coils. Eight of the circuits include pairs of spiral coils. The single coils generate non-uniform fields. The non-uniform fields generated by the single coils are generated mostly in the Z direction at the sweet spot. Two coils in a pair of spiral coils are positioned side-by-side. The coils are connected in series. Opposing coils connected in series produce non-uniform fields pointed mostly in the X and Y directions at the sweet spot. A single large coil generates a mostly uniform Z field. A pair of long narrow spirals on opposite edges of the PCB generates a mostly uniform X field. Another pair of long narrow spirals on the other pair of opposite edges of the PCB generates a mostly uniform Y field.

The PCB utilizes an approximate nature of the “mostly uniform” fields to produce an effect of the desired “varying mostly” fields. The “mostly uniform” fields may have gradients. For example, consider the Z-direction fields. One large coil generates a “mostly uniform” Z field. Three small coils may be placed near the origin of the PCB and offset from the origin along lines at roughly 0 degrees, 120 degrees, and 240 degrees. The three small coils generate smaller “mostly uniform” Z fields displaced from the main “mostly uniform” Z field generated by the large coil. The effects of the “mostly varying” fields may be produced by taking sums and differences among the four fields discussed above. Fields in the X and Y directions may be generated similarly. However, connected pairs of series-opposing coils may be used instead of single coils to generate fields in the X and Y directions. The above fields may be calculated using a straight line segment field model, for example.

In an embodiment, the tracker electronics 130 includes twelve receiver coil drivers. The twelve coil drivers operate at twelve different CW frequencies, for example. The twelve coil drivers drive twelve receiver coil circuits on the receiver PCB. Currents in the twelve receiver coil circuits are measured. In an embodiment, current values are approximately determined. Then, ratios of the currents are determined.

Current in the coils causes the receiver coil circuits to emit magnetic fields. The magnetic fields induce voltages in a single transponder coil at the twelve driver frequencies. The tracker electronics 130 measures signals at the twelve frequencies.

A mutual inductance between each receiver circuit and the transponder coil is calculated. Mutual inductances between the transponder 115 and receiver 120 are determined. In an embodiment, mutual inductances are approximately determined. Then, ratios of the twelve mutual inductances are determined. Six or more receiver coils spread in a selected configuration and measurements of the ratios of the mutual inductances to the transponder coil may be used to calculate a position of the transponder coil, an orientation (except roll) of the transponder coil, and a gain of the transponder coil (a POG determination). The gain of the transponder coil represents a scale factor that converts the mutual inductance ratios into mutual inductance values (in Henries, for example).

In an alternative embodiment, a single-receiver-coil version PCB may be used to characterize three coils in an ISCA receiver or transponder coil trio. The characterization process includes separately tracking each of the three ISCA coils for position, orientation, and gain. Then, the tracking data are combined into a coil characterization format used by ISCA trackers, for example.

FIG. 3 depicts a flow diagram for a method 300 for a POG determination used in accordance with an embodiment of the present invention. First, at step 310, receiver coils are driven at different frequencies. Drivers produce currents in the receiver coils. Then, at step 320, ratios of the currents produced in the receiver coils are determined. The receiver coils generate magnetic fields that induce voltages at different frequencies in the transponder coil. At step 330, the signals induced at the transponder coil are measured.

The voltages and currents produce mutual inductances between the transponder coil and the receiver coils. At step 340, ratios of the mutual inductances between the receivers and the transponder are calculated.

Next, at step 350, an initial estimate, or seed, of transponder position, orientation, and gain is obtained. The estimate may be generated from prior mechanical knowledge of the transponder POG, from a final POG estimate from a previous tracking cycle, or from a direct calculation from the mutual inductance measurements, for example.

Then, at step 360, a best-fit estimate of the POG to the mutual inductance ratio measurements may be calculated. The best-fit estimate may be calculated using a model of the transponder-to-receiver mutual inductances and the seed POG values, for example. The best fit calculation may be any of several well-known solution fitting algorithms, such as least squares, Powell, and Levenberg-Marquardt, for example.

The above calculations may also be performed with the PCB configured as a twelve transponder coil board with a single receiver coil. Additionally, the PCB may be configured with different numbers of coils to function as a transponder and/or receiver.

In an embodiment, electromagnetic tracking systems calibrate receiver electronics to help ensure accurate positional measurements, for example. A calibration coil may be placed diagonally in a receiver coil assembly to provide approximately equal mutual inductances from the calibration coil to each of the receiver coils. The mutual inductances may be individually measured during manufacture. The mutual inductance values measured during manufacture may be stored in a characterization memory, for example. The measured mutual inductances may be used during tracking to calibrate the receiver electronics.

The PCB may include a calibration coil. The calibration coil may improve the usefulness of the PCB as a receiver 120. In an embodiment, the calibration coil is built on an inner layer or layers of the printed circuit assembly. The calibration coil may partially overlap existing coils in the assembly to produce desired calibration coil to receiver coil mutual inductances. In an embodiment, a single-turn calibration coil in a rectangle covering approximately one corner quadrant of the PCB is used.

In an embodiment, the calibration coil is part of a single PCB, rather than a separately fabricated addition. Thus, the calibration coil is in approximately the same plane as the receiver coils. Mutual inductances between the calibration coil and the receiver coils may be fixed by a fabrication process and calculated without measuring separate boards, for example. Alternatively, a separate calibration module may be added to measure small mutual inductances or mutual impedances separate from the coil assembly.

Ratios of transponder 115 currents to a reference current in the calibration coil may be determined, for example. The calibration coil may have a defined mutual inductance with respect to each receiver coil. The mutual inductances, combined with measured current ratios, allow determination of transponder-to-receiver mutual inductances from the measured ratios. If a wireless transponder is used, current ratios may not be measured. Another measurement, such as magnetic field ratios, may be used with wireless transponders.

Thus, certain embodiments of the PCB provide transponder and receiver coils that do not need precise characterization. Certain embodiments use pairs of coils in series to generate magnetic fields parallel to the plane of the PCB while reducing the number of separate coil drivers used. For the ANT-009 coil board, 12 drivers are used. A separate-coil version of the ANT-009 may use 20 drivers. Additionally, the straight line segments of the PCB allow use of an analytical model of a magnetic field due to a straight line current segment. Furthermore, expressions for mutual inductance between two straight line current segments may be used. Certain embodiments of the PCB also provide for calibration of the receiver and tracker electronics.

Certain embodiments of the present invention provide an electromagnetic tracking system including a wired or wireless transponder with a single-coil. In an embodiment, one receiver coil assembly, whether PCB or wire-wound, may be used to simultaneously track a plurality of wireless and/or wired transponders on different frequencies.

Additionally, a pre-operative plan may be entered into a surgical navigation workstation prior to performing a medical procedure. The plan may be transcribed to a surgical navigation workstation and followed during the course of the medical procedure by guiding a medical instrument with a transponder and receiver. For example, therapeutic materials may be associated with a particular treatment placement device and a path to be traversed by the placement device inside a patient may be entered into the system. During the procedure, the placement device can be manipulated by observing an image of the patient's anatomy and the predetermined path, and guiding the placement device using a transponder attached to the device so that it follows the predetermined path. An image of the placement device following the predetermined path can be displayed on a monitor or other display device. Upon reaching the predetermined destination within the patient's anatomy, the therapeutic materials can be dispersed in accordance with the pre-operative plan.

The pre-operative plan can be updated with real-time feedback during the course of the procedure to allow for changes in the plan to be accommodated. For example, the placement of radioactive seeds for such interventions as brachytherapy on the prostate can be entered into the system as a pre-operative plan. During the procedure, changes in anatomy or other conditions can be accommodated for by allowing real-time feedback from a surgeon to adjust the path of the medical instrument being guided within the patient

This approach may be used, for example, in a form of cancer therapy using radioactive seeds. The planning could be done in both 2D and 3D. With 3D, the isocontours of the radiation plan could be checked against actual placement.

With this approach, placement and retrieval of the seeds could be accomplished through navigation. Conventionally, this is typically done by combining x-ray and ultrasound. X-rays are used to see the seeds but not the soft tissue and ultrasound to see the soft tissue and not the seeds. It is challenging is to register the two conventional modalities. This is particularly so because the perspective and distortion are different in each conventional modality and it is difficult to pick up common landmarks. However, this new approach overcomes those drawbacks of the conventional approach.

FIG. 5 depicts a flow diagram for a method 500 for improved instrument tracking in a surgical navigation system. This method 500 may be performed in software, hardware, or a combination thereof. For ease of understanding, this method is delineated as separate steps represented as independent blocks in FIG. 5; however, these separately delineated steps should not be construed as necessarily order dependent in their performance.

This described method 500 includes storing (at 502) a plan for an image-guided procedure, before conducting the procedure. The plan includes a path to be traversed by a medical instrument during the procedure. The described method also includes displaying (at 504) an image of a patient's anatomy on a display and superimposing a pictorial representation of the path on the image. In addition, the described method also includes receiving (at 506) a transponder signal at a receiver. The transponder being coupled to a medical instrument inside a patient's body and emitting said transponder signal. Furthermore, the described method includes determining (at 508) a position of the transponder based, at least in part, upon the transponder signal.

FIG. 6 depicts a flow diagram for a method 600 for adjusting the plan for an image-guided procedure while the procedure is occurring, especially where such procedure involves exposure to radiation. This method 600 may be performed in software, hardware, or a combination thereof. For ease of understanding, this method is delineated as separate steps represented as independent blocks in FIG. 6; however, these separately delineated steps should not be construed as necessarily order dependent in their performance.

This described method 600 employs the techniques described above in method 500 and shown in FIG. 5. This described method 600 includes comparing (at 602) a planned radiation exposure dose of a planned image-guided procedure to an actual radiation exposure dose during the actual image-guided procedure. Of course, as part of the comparison the actual exposure does is measured. In response to the comparisons, the plan is updated (at 604). The actual may vary from the plan for many reasons. For example, the practitioner may have to place the seeds in different locations or take a different path then what was planned because, for example, of aspects of the anatomy not known at the time of planning. This is much like an aircraft would alter is flight path in response to real-time conditions such as weather.

With the updated plan, the practitioner would then have the ability to check the accuracy of the plan, with its optimal radiation dose, to the actual radiation dose, and would therefore have additional information from which to assess the potential clinical outcome of the treatment. For instance, if the actual placement resulted in a lesser dose being delivered to the treatment site, the practitioner could recommend an extension to the overall treatment plan prior to retrieval of the seeds. Premature removal of the seeds could result in sub-optimal dosing of the treatment site, yielding a less effective treatment.

Attempting to gauge the effectiveness of placement through imaging (X-ray and/or ultrasound) might not allow for a true or accurate 3D positioning of the seeds and an inaccurate comparison between planned and actual placement. Again, the result may be less than optimal treatment (either in the form of too little dose to the treatment site or too much dose to the healthy tissue).

While the invention has been described with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from its scope. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed, but that the invention will include all embodiments falling within the scope of the appended claims. 

1. A method for improved instrument tracking, said method comprising: in a surgical navigation system, storing a plan for an image guided procedure, before conducting said procedure, said plan comprising a path to be traversed by a medical instrument during said procedure; displaying an image of a patient's anatomy on a display and superimposing a pictorial representation of said path on said image; receiving a signal at a receiver from a transponder, said transponder being coupled to a medical instrument inside a patient's body and emitting said transponder signal; determining a position of said transponder based, at least in part, upon said transponder signal.
 2. The method of claim 1 further comprising transmitting data from said transponder to said receiver.
 3. The method of claim 2 wherein said data comprises information identifying said transponder.
 4. The method of claim 3 wherein said data distinguishes said transponder from other transponders.
 5. The method of claim 1 wherein said plan comprises the administration of a therapeutic material.
 6. The method of claim 5 further comprising facilitating administration of said therapeutic material when said medical instrument has traversed said path.
 7. The method of claim 1 further comprising determining said plan for said image guided procedure before conducting said procedure.
 8. The method of claim 1 further comprising: facilitating insertion of said medical instrument inside a patient's body; facilitating guiding of said medical instrument along said path inside the patient's body so that the transponder substantially traverses said path.
 9. The method of claim 1 further comprising superimposing an indication of said position of said transponder on said image.
 10. The method of claim 1 further comprising superimposing an indication of said position of said transponder on said image with said superimposed pictorial representation of said path on said image.
 10. The method of claim 1, wherein said plan further comprises a planned radition exposure dose, the method further comprising: measuring an actual radiation exposure dose during an actual image-guided medical procedure; comparing the planned radiation exposure dose of a planned image-guided procedure to the actual radiation exposure dose during the actual image-guided procedure; updating the plan for an image guided procedure in response to the comparison.
 11. One or more processor-readable media having processor-executable instructions that, when executed by a processor of a surgical navigation system, performs acts comprising: in a surgical navigation system, storing a plan for an image guided procedure, before conducting said procedure, said plan comprising a path to be traversed by a medical instrument during said procedure; displaying an image of a patient's anatomy on a display and superimposing a pictorial representation of said path on said image; receiving a signal at a receiver from a transponder, said transponder being coupled to a medical instrument inside a patient's body and emitting said transponder signal; determining a position of said transponder based, at least in part, upon said transponder signal.
 12. One or more media as recited in claim 11 further comprising determining an orientation of said transponder based, at least in part, upon said transponder signal.
 13. One or more media as recited in claim 11 further comprising: deriving data from said transponder; identifying said transponder based upon data derived from said transponder.
 14. One or more media as recited in claim 13, wherein said data distinguishes said transponder from other transponders.
 15. One or more media as recited in claim 11 wherein said plan comprises the administration of a therapeutic material.
 16. One or more media as recited in claim 11 further comprising facilitating administration of said therapeutic material.
 17. One or more media as recited in claim 11 further comprising facilitating determination of said plan for said image guided procedure before conducting said procedure.
 18. One or more media as recited in claim 11 further comprising: facilitating insertion of said medical instrument inside a patient's body; facilitating guiding of said medical instrument along said path inside the patient's body so that the transponder substantially traverses said path.
 19. One or more media as recited in claim 11 further comprising superimposing an indication of said position of said transponder on said image.
 20. One or more media as recited in claim 11 further comprising superimposing an indication of said position of said transponder on said image with said superimposed pictorial representation of said path on said image. 